recordable: add startRecording method

This commit adds a new startRecording method to the Recordable class.
This method can be used to start a recording and takes advantage of
MediaRecorder's new API (no call to setRecordingPath). Also,
startRecording is meant to be passed the path directly instead of
using initRecFilename which is removed in this commit.

In addition to these changes we modify the Recordable class to operate
on a single recorder_ object created at construction time instead of
creating/destroying it in toggleRecording and stopRecording.

Change-Id: Icf170e2a0ba5bbbb34afa37a54cf5edc76c8338d
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>